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Substrate Preparation Recommendations

The most critical aspect of protecting timber is the preparation of the surface prior to the application of coatings and the key areas are as follows.

General Cleaning
Often neglected, a thorough cleaning of the surface will remove most of the contaminants which can impair the absorption, adhesion and subsequent performance of coating systems. A solution of household detergent in warm water and a stiff, non-metallic brush is ideal for removing oil, dust, dirt or grease. Thoroughly rinse off any residues and allow the surface to dry fully.

Fungicidal Wash
Algae and mould growth not only impair adhesion of coatings, they also have the potential to grow through them and damage the film itself. Treat growths with a fungicidal wash or bleach solution and allow 15-20 minutes to work. Then loosen the dead spores using a stiff, non-metallic bristle brush, rinse with clean water to remove all residues and allow the surface to dry thoroughly.

Removal of old Coatings
Sanding is the most practical way to remove small amounts of unsound coating while electric sanders or even chemical strippers are better for large areas. Do not remove sound coatings, particularly opaque finishes, for the sake of it, but do test them to make sure they are truly sound. Wood stains must be removed if a lighter shade is required.

Remember that base stains used on new joinery provide approximately 3 months protection, at best. Assess its condition before top coats are applied, otherwise their performance is unpredictable.

Sanding
Bare timber exposed to direct sunlight for more than 3 weeks will be damaged to the point where removal of the surface back to bright timber is necessary. Mechanical sanding is the only really effective method. Abrasive papers should be used - DO NOT use wire brushes or steel wool. Metal fragments can become lodged in the timber where they can discolour (and/or rust) and eventually damage the coating.

Cut out Decay
Hyphae, which are part of the fungal body which causes decay, can reach some distance into the timber. It is not sufficient to simply remove soft timber; to ensure that all the infestation is removed cut up to an inch (2.5cm) into sound timber.

Preservative Treatment
If damaged timber has been removed from a rotten area, decay is likely to occur again. In this situation a preservative should be applied to saturation. If timber is used to splice in for repair, this should either be of a durable species or pre treated with preservative before use.

Degreasing
This refers to the use of a solvent to remove surface contaminants. These include natural timber components such as tannins, gums and oils from hardwoods and resins from softwoods. Clean with a cloth dampened with solvent (methylated spirits or cellulose thinners, but NOT white spirit or turps) and change the face of the cloth regularly to avoid contamination.

Knots
Play a hot air gun over live knots to draw out as much resin as possible, scrape off the resin and degrease the timber as described previously. Remove dead knots before they fall out and fill the void with a suitable wood filler.

Arrises
To ensure an even, adequate coating can be applied to vulnerable edges, all cills should be rounded to approximately 3mm radius (pencil round") and all other sections to at least 1 to 2mm radius.

Silicone Contamination
Silicone creates serious problems if not used carefully. It is usually recognisable as cissing / cratering / fish-eyeing in the applied coating, and is similar to trying to paint waxed paper. Contamination cannot be removed by sanding, washing or with any of the usual solvents, only by the use of a silicone remover or "digester". Multiple applications may be necessary for complete removal.

Mortar / Iron / Water Staining
Mortar and iron staining are unsightly, and mortar can cause serious breakdown of the coating. The best treatment is with a wood bleach (active ingredient - oxalic acid). This is a hazardous chemical so take great care when using it. Remove all of the surface coating prior to carrying out treatment. Where iron staining has occurred remove the original cause of the discolouration. Visible signs may be removed with a Unsaturated solution of oxalic acid. This can be purchased in powder form from 81 chemists. Acid two liberal tablespoons to 500 ml of warm water and mix thoroughly. Apply to the affected areas using a synthetic bristle brush. Leave for 30 minutes. Rinse thoroughly with clean water and allow to dry. Eye and hand protection is strongly advised when using this solution.

Nails Screws
Non-ferrous fixings are strongly recommended. If the use of steel / iron fixings is unavoidable, they should be driven home and the resultant holes filled with Cetol Gupa wood filler

Moisture Content
When using timber which is to be coated, its moisture content should be close to what it will be in service. Below 14% for interior timber and between 15-18% for exterior timber is recommended.

Fillers should be used to fill any voids or areas of damage in the timber prior to coating - see fillers

Salt Efflorescence
This not only affects brickwork, it also occurs in plywood when atmospheric moisture causes the salts in the glue line to migrate to the surface. They appear as a white crystalline powder and while it in no way affects the strength or integrity of the plywood, it is unsightly. Normally, rainwater washes the salts away However, on sheltered areas such as soffits, they remain as a 'bloom on the surface. The remedy is to wash down the affected areas with clean water and a bristle (non-metallic) brush without over-scrubbing. The surface can then be re-treated. The phenomenon, which varies with the type of plywood and country of origin, may recur in the future but it will be to a lesser extent and again, removal by washing will I be required . The amount of salt in the glue line is very variable, but finite, so efflorescence will cease with time.